dApp Development Interview Questions for Blockchain

Browser wallets inject an EIP-1193 provider at window.ethereum. Wrap it with BrowserProvider and request accounts. import { BrowserProvider } from 'ethers'; async function connect() { if (!window.ethereum) throw new Error('Install a wallet'); const provider = new BrowserProvider(window.ethereum); const signer = await provider.getSigner(); // prompts the user const address = await signer.getAddress(); const { chainId } = await provider.getNetwork(); return { signer, address, chainId }; } In production, use EIP-6963 discovery or a connector library so users with several wallets can choose.

Create a Contract with address, ABI and a provider for reads or a signer for writes. import { Contract, parseEther } from 'ethers'; const abi = [ 'function count() view returns (uint256)', 'function increment() payable' ]; const readOnly = new Contract(ADDR, abi, provider); console.log(await readOnly.count()); // free eth_call const writable = new Contract(ADDR, abi, signer); const tx = await writable.increment({ value: parseEther('0.001') }); const receipt = await tx.wait(); console.log(receipt.hash, receipt.status);

curl -L https://foundry.paradigm.xyz | bash && foundryup forge init my-dapp # new project with src, test, script forge install OpenZeppelin/openzeppelin-contracts forge build # compile forge test -vvv # run tests with traces forge coverage # coverage report forge snapshot # gas snapshot anvil # local node cast call ADDR 'count()(uint256)' --rpc-url $RPC Foundry tests are written in Solidity and run fast; Hardhat is the JavaScript or TypeScript alternative with a large plugin ecosystem.

Verification uploads source and compiler settings so the explorer can recompile and match the bytecode, letting users read the code. # Foundry, using an explorer API key forge verify-contract 0xDeployedAddr src/Counter.sol:Counter \ --chain sepolia --etherscan-api-key $KEY --watch # With constructor arguments --constructor-args $(cast abi-encode 'constructor(address)' 0xOwner) # Hardhat npx hardhat verify --network sepolia 0xDeployedAddr 0xOwner Compiler version, optimiser runs and via-IR settings must match exactly.

Use contract.on for live events and queryFilter for history. const abi = ['event Transfer(address indexed from, address indexed to, uint256 value)']; const token = new Contract(TOKEN, abi, provider); token.on('Transfer', (from, to, value, event) => { console.log(from, to, value.toString(), event.log.blockNumber); }); const filter = token.filters.Transfer(null, MY_ADDRESS); const past = await token.queryFilter(filter, -5000); // last 5000 blocks console.log(past.length); Use a WebSocket provider for live events, and page large ranges because providers limit block spans.

viem is a typed, lightweight alternative to ethers, and wagmi builds React hooks on it. import { createPublicClient, createWalletClient, http, custom } from 'viem'; import { sepolia } from 'viem/chains'; const pub = createPublicClient({ chain: sepolia, transport: http() }); const count = await pub.readContract({ address: ADDR, abi, functionName: 'count' }); const wallet = createWalletClient({ chain: sepolia, transport: custom(window.ethereum) }); const [account] = await wallet.requestAddresses(); const hash = await wallet.writeContract({ address: ADDR, abi, functionName: 'increment', account }); await pub.waitForTransactionReceipt({ hash });

// script/Deploy.s.sol pragma solidity ^0.8.24; import {Script, console} from 'forge-std/Script.sol'; import {Counter} from '../src/Counter.sol'; contract Deploy is Script { function run() external returns (Counter c) { vm.startBroadcast(); c = new Counter(); vm.stopBroadcast(); console.log('Counter deployed at', address(c)); } } # Run it with an encrypted keystore account forge script script/Deploy.s.sol --rpc-url $RPC_URL \ --account deployer --broadcast --verify Broadcast output in the broadcast folder records addresses for later scripts.

Anvil or Foundry tests can fork a live chain at a block, so you interact with real tokens and protocols locally. # Start a local fork anvil --fork-url $MAINNET_RPC --fork-block-number 21000000 # Or fork inside a test function setUp() public { vm.createSelectFork(vm.envString('MAINNET_RPC'), 21000000); } # Give yourself tokens or impersonate a whale deal(address(usdc), alice, 1_000e6); vm.prank(whale); Pinning the block number keeps results reproducible and makes RPC caching effective.

Check chainId after connecting and listen for chainChanged. Then ask the wallet to switch, and add the chain if it is unknown. const target = '0xaa36a7'; // Sepolia 11155111 try { await window.ethereum.request({ method: 'wallet_switchEthereumChain', params: [{ chainId: target }] }); } catch (e) { if (e.code === 4902) { await window.ethereum.request({ method: 'wallet_addEthereumChain', params: [{ chainId: target, chainName: 'Sepolia', rpcUrls: [RPC], nativeCurrency: { name: 'ETH', symbol: 'ETH', decimals: 18 } }] }); } } Disable write buttons until the network is correct.